Additional fixes for 10.6:
fts_sync_commit(): Release cache->lock also on rollback.
fts_sync_write_words(): Avoid a crash if an error occurs,
by stopping at the first error.
fts_add_doc_by_id(): Sync the doc id only after adding the doc id
to the cache.
- During shutdown, InnoDB fts fails to update synced doc id
when there is only one doc id about to sync. While starting
the server, InnoDB fetches the already synced doc id from
config table. In the subsequent sync operation, InnoDB fails
with DB_DUPLICATE_KEY error.
